In numerous industrial techniques and processes, it is often desirable to prevent the shifting of adjacent surfaces with respect to one another. One of these industrial techniques can be found in the manufacture of closures, such as doors or hoods for automobiles or other transportation vehicles. These closures require a double layer of steel which is typically formed by joining two planar surfaces or panels of steel. An inner panel is positioned within an outer panel with the edge of the outer panel extending beyond the edge of the inner panel. The edge of the outer panel is then folded or crimped over the edge of the inner panel in a process known as hemming, and the resulting structure is known as a hem flange.
In order to permanently secure the inner panel with respect to the outer panel of a hem flange, an adhesive is applied between the overlapping metal surfaces and cured, or the two panels are welded together to form the permanent hem-flange assembly. After formation of the hem flange and just prior to any permanent adhesive cure or welding, the hem flange is transported and handled, and shifting of the panels with respect to one another often occurs, which can result in numerous problems in the manufacturing process. It is therefore desirable to utilize an adhesive composition having some initial green strength or handling strength or to utilize other means so as to obtain some degree of immediate handling strength to prevent shifting of the panels just prior to permanent cure or welding.
Various adhesive compositions have been developed in an attempt to provide the initial handling strength desirable in the formation of hem flanges. Examples of these adhesive compositions can be found in, for example, U.S. Pat. Nos. 4,703,809; 4,855,001; 4,857,131; and 5,096,962. It has proven to be relatively difficult to formulate a one-part adhesive composition having both high handling strength and sufficient stability or shelf-life characteristics. Most adhesive compositions having adequate handling strength and shelf life are only available in two-part formulations which require expensive application techniques and equipment and require that panels be mated within a short period of time after mixture and application of the two parts of the adhesive.
Another method of providing initial handling strength to hem flanges is the utilization of induction-cure equipment in combination with an adhesive composition. The induction-cure equipment is used to heat up the metal panels of the hem flange to pre-gel the adhesive composition so as to obtain handling strength. However, induction-cure equipment is expensive and burdensome.
When utilizing adhesive compositions in the formation of hem flanges, it has also been proposed to employ spherical glass beads in order to maintain a consistent thickness of the adhesive composition applied between the metal panels. The glass beads act as spacers and prevent the metal panels from squeezing out the adhesive which could result in adhesive bond failure.